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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Thank you! I am doing a structural optimization filter that inherently cannot be parallelized.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Vysakh<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b>From:</b> Barry Smith <bsmith@petsc.dev> <br>
<b>Sent:</b> Tuesday, January 17, 2023 3:28 PM<br>
<b>To:</b> Venugopal, Vysakh (venugovh) <venugovh@mail.uc.edu><br>
<b>Cc:</b> petsc-users@mcs.anl.gov<br>
<b>Subject:</b> Re: [petsc-users] about repeat of expensive functions using VecScatterCreateToAll<o:p></o:p></p>
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<p><strong><span style="font-family:"Open Sans",sans-serif;color:white;background:#E00122">External Email: Use Caution</span></strong><span style="color:white"><o:p></o:p></span></p>
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<p class="MsoNormal">On Jan 17, 2023, at 3:12 PM, Venugopal, Vysakh (venugovh) via petsc-users <<a href="mailto:petsc-users@mcs.anl.gov">petsc-users@mcs.anl.gov</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Hi,</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">I am doing the following thing.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"> </span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Step 1. Create DM object and get global vector ‘V’ using DMGetGlobalVector.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Step 2. Doing some parallel operations on V.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Step 3. I am using VecScatterCreateToAll on V to create a sequential vector ‘V_SEQ’ using VecScatterBegin/End with SCATTER_FORWARD.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Step 4. I am performing an expensive operation on V_SEQ and outputting the updated V_SEQ.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Step 5. I am using VecScatterBegin/End with SCATTER_REVERSE (global and sequential flipped) to get V that is updated with new values from V_SEQ.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Step 6. I continue using this new V on the rest of the parallelized program.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Question: Suppose I have n MPI processes, is the expensive operation in Step 4 repeated n times? If yes, is there a workaround such that the operation in Step 4 is performed only once? I would
like to follow the same structure as steps 1 to 6 with step 4 only performed once.</span><o:p></o:p></p>
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<p class="MsoNormal"> Each MPI rank is doing the same operations on its copy of the sequential vector. Since they are running in parallel it probably does not matter much that each is doing the same computation. Step 5 does not require any MPI since only part
of the sequential vector (which everyone has) is needed in the parallel vector.<o:p></o:p></p>
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<p class="MsoNormal"> You could use VecScatterCreateToZero() but then step 3 would require less communication but step 5 would require communication to get parts of the solution from rank 0 to the other ranks. The time for step 4 would be roughly the same.<o:p></o:p></p>
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<p class="MsoNormal"> You will likely only see a worthwhile improvement in performance if you can parallelize the computation in 4. What are you doing that is computational intense and requires all the data on a rank?<o:p></o:p></p>
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<p class="MsoNormal">Barry<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Thanks,</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"> </span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Vysakh Venugopal</span><o:p></o:p></p>
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<p class="MsoNormal" style="background:white"><span style="font-family:"Arial",sans-serif;color:black;border:none windowtext 1.0pt;padding:0in">---</span><o:p></o:p></p>
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<p class="MsoNormal" style="background:white"><span style="font-family:"Arial",sans-serif;color:black;border:none windowtext 1.0pt;padding:0in">Vysakh Venugopal</span><o:p></o:p></p>
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<p class="MsoNormal" style="background:white"><span style="font-family:"Arial",sans-serif;color:black;border:none windowtext 1.0pt;padding:0in">Ph.D. Candidate</span><o:p></o:p></p>
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<p class="MsoNormal" style="background:white"><span style="font-family:"Arial",sans-serif;color:black;border:none windowtext 1.0pt;padding:0in">Department of Mechanical Engineering</span><o:p></o:p></p>
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<p class="MsoNormal" style="background:white"><span style="font-family:"Arial",sans-serif;color:black;border:none windowtext 1.0pt;padding:0in">University of Cincinnati, Cincinnati, OH 45221-0072</span><o:p></o:p></p>
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